Calibration of logging systems



March 31, 1953 c. w. ZIMMERMAN CALIBRATION OF LOGGING SYSTEMS 2 SHEETSSHEET 1 Filed July 50, 1951 SOURCE 9 3 m a w, 2 f Ir z wing? 2.1; a m V E w dw I u W L 1 1T j I W CAEL V14 Z lMMEQA IAN INVENTOR.

BY AGE/VT Patented Mar. 31, 1953 CALIBRATION OF LOGGING SYSTEMS Carl W. Zimmerman, Dallas, Tex., assignor, by mesne assignments, to Socony-Vacuum Oil Company, Incorporated, New York, N. Y., a

corporation of New York Application July 30, 1951, SerialNo. 239,371

14 Claims.

This invention relates to the calibration of lo ging systems in which investigations of the character of, or conditions in, formations adjacent a well bore proceed through the application of constant frequency sinusoidally varying forces.

In well logging systems characterized by the production of an electrical signal having two components respectively dependent upon different qualities of the formations adjacent a well bore or conditions in the formations, a problem is initially to adjust the system so that the two effects or components may be properly separated.

For example, in induction logging systems of the type illustrated by the patent to Broding No. 2,535,666, formations adjacent the well bore are electromagnetically coupled to a coil system and the resultant signal is phase detected for the production of two components, a first dependent upon conductivity of the formations and a second dependent upon the magnetic susceptibility of the formations. While it is desirable to make all initial adjustments before the exploring sonde is lowered into the bore hole, the presence of large amounts of magnetic materials in structures at the well head is so disturbing that such calibration may not be made. The calibration must therefore be made either at a remote point before the instrument is lowered into the well bore or some means must be provided for control of the necessary adjustments after the instrument is lowered to a depth beyond the influence of the otherwise disturbing magnetic bodies.

Similarly, in acoustic logging systems of the type generically illustrated by the patent to Kean No. 2,530,971, a signal is produced dependent upon the acoustic impedance of formations. Such a signal may be separated by phase detecting means into two components which correspond with acoustic resistance and reactance of the formations. For proper separation of the signal into two components a calibration or initial adjustment problem is present.

.In still a third type logging system, such as illustrated by applicants copending application, Serial No. 189,818 entitled A Dielectric Well Logging System, a similar problem is present.

It is an object of the present invention to provide a system for readily calibrating loggin systems of the type utilizing a formation-dependent signal separable into two components each independently related to distinct characteristics of the formation.

Ina preferred form of the invention calibration meansis provided for a system having a formation exploring element movably positioned in a well bore and energized from an alternating current source to produce a formation-dependent signal for application to a first of a pair of inputcircuits of a phase detecting network. An impedance having a dominant characteristic equivalentto a selected property of said formation is coupled to the formation exploring element by means including circuit completing means at the'surface'of the earth to produce an added signal component at the first input circuit identical in character to the component produced by said selected property; and a circuit is provided for transmitting from the source to the second of the input circuits a reference voltage in phase-coincidence with the added signal component.

For a more complete understanding of the present invention and for further objects'and adva tages thereof, reference may now be had to the following description taken in conjunction with the accompanyingdrawings in which:

Fig. 1 illustrates calibration of an induction logging system;

Fig. 2 illustrates calibration of an acoustic logging system; and

Fig. 3 is a modification of the system of .Fig. 2.

Referring now to Fig. 1, there is illustrated a formation exploring element l6 comprising an elongated solenoid forming one arm of a Maxwell bridge network l I. The bridge network H is excited through transformer Ha and, by way of a circuit l2 forming a part of a cable ['3, from a power source It at the surface of the earth. The elements below the dotted loop, representing the outline of cable i3, are to be mounted in a suitable housing, not shown, and movablypositionedin a well bore in the manner generally indicated in Patent No. 2,535,666 to Robert A. Broding.

The vertical or output diagonal of the bridge I! is connected through a relay arm [5 to one terminal of transformer l6 and by way of conductors I? to the second terminal of transformer 16. The bridge output signal may thereby be transmitted to an amplifier It. The amplifier output circuit I 9 extends from the exploring unit to the'surface of the earth and is connected by way of transformer 26 to one input circuit of phase detecting network 2 l The-circuit l9-is also connected byway of transformer 22 to one input circuit of a second phase detecting network 23. The phase-detecting networks 2i and 23 have associatedtherewith-011tput indicating devices generically representedxby meters 24 and25. Theoutput'of the alternatm' g current source I4 "is also connected to the-phase detecting networks 2.! and E23 :by *way of a phase ad usting circuit =26. The output of phase'adju'sting network 26 is connected by way of circuit 21 to a second input circuit at transformer 23 in network 2| to apply a reference signal thereto. Through a second phase adjusting network 29, a reference signal is applied to a transformer 30 at a second in ut circuit of a network 23.

In operation, the bridge network I l and its formation exploring element It are lowered to the bottom of a bore hole. The phase of the reference signal applied by way of channel 21 to transformer 28 is adjusted as to be 90 out of phase with the reference signal applied by circuit 29 to the transformer 30. If the phase of the output of circuit 26 is then properly adjusted with respect to the bridge exciting signal on channel I2, meter 24 will indicate variations in the electrical conductivity of formations or media adjacent the formation coupling i independently of magnetic susceptibility and the meter 25 will indicate the magnetic susceptibility of formations independzently of electrical conductivity.

By applicants invention, the formation exploring element In at the bottom of a bore hole and remote from iron masses may be calibrated by adjusting the phase angle between the signals on channels [2 and 21 for optimum operation.

More particularly, applicant provides a switching circuit including conductor 35 in cable l3 which extends into the bore hole from the surface. At the surface of the earth conductor 35 is connected to the movable arm of a multi-terminal switch 36. A battery 31 having its negative terminal connected to the first switch contact is connected at its positive terminal to conductor l9a which extends into the bore hole. The sec- .ond terminal of switch 36 is open-circuited. The

third terminal is connected to the positive pole of a battery 38. The negative terminal thereof is connected to conductor l9a. The fourth terminal of switch 36 is connected to the positive terminal of a battery 39 whose negative terminal is connected to conductor l9a.

Adjacent the exploring element I 0, the conductor 35 is connected through a first rectifier 46 and the coil of a relay 4| to conductor 11. Conductor IT is connected by way of conductor 42 to conductor So to complete a conductive circuit to switch 36. Conductor 35 is also connected by way of a second rectifier 43 and the coil of a second relay 44 to conductor i1.

By actuation of relays 4| or 44, a resistor 45 or a condenser 46 selectively may be coupled to the formation exploring element Id. The resistor 45 is connected between the armature of relay ti and one terminal of a coil 41. The coil 47 is wound around or inductively linked to the solenoid Ill. The other terminal of coil 41 is connected to the normally open contact of relay 4!. Similarly, condenser 46 is connected between the armature of relay 44 and the first terminal of the coil 41. A third relay is connected by way of conductor 5| to conductor l1 and by way of resistor 52 to conductor 35. The normally open terminal of the relay 5!!- is connected by way of conductor 53 to a resistor-capacitor circuit 54 which is connected across circuit l2.

The individual impedances in circuit 54 are selected so that the signal between conductor 53 and ground has the same character both in amplitude and phase as the signal from the bridge II when the coil I6 is in air and remote from an inductive or low resistive bodies. Resistor 45 preferably is non-inductive and condenser 46 preferably has infinite resistance. I

In actual calibration procedure the switch 36 may be placed in its first circuit completing position whereby the voltage from battery 31 appears between conductors I! and 35. In the second position of switch 36 the bridge unbalance signal, separated into two components of generally unknown phase relation appears on meters 24 and 25. The voltage on channel l'l-35 in switch position 1 is insufficient to actuate relay 50 but, by producing flow of current through rectifier 43, actuates relay 44. Relay 4! is not affected because it is isolated by the rectifier all. When relay 44 is energized, condenser 46 is connected in circuit with the coupling coil 4? to introduce in the bridge network I! an added unbalance effect identical in nature with that produced by a change in susceptibility of the media adjacent thereto. Stated otherwise, a component is added to the unbalance signal from bridge i i by reason of the presence of condenser 46 in the coupled circuit that has the same phase relation as the component present by reason of the fact that the formation has a magnetic susceptibility different than that of air. The variable phase circuit 26 is then adjusted so that, for repeated actuation of switch 36 from position 1 to position 2, meter 24 senses no change. This condition is established when the phase angle between the reference signal applied to transformer 28 is in phase-quadrature with the component of the signal applied to transformer 26 that is due to the presence of condenser 46 in the coupled circuit and is in phase-coincidence (in-phase or out-of-phase) with the component of bridge unbalance signal due to conductivity of formations. Regardless of the character of the formation adjacent the solenoid l6 and the consequent initial bridge unbalance, the addition of the condenser increases but one of the two components of the bridge unbalance, giving a reliable guide for proper adjustment of the phase shifting circuit 26.

In a similar manner when the switch 36 is in position 3, current fiows through rectifier 40 and the coil is relay 4! thereby connecting the resistor 45 in circuit with coil 41. If the phase adjusting circuit 26 was properly set during the previously described operation, meter 24 will sense the presence or absence of resistor 45. The phase adjusting circuit 29, identical with that of circuit 26, is then adjusted so that meter 25 senses no change upon the insertion of the resistor 45. If the switch is then moved to position 4., there is applied to conductors I1 and 35 the voltage of the battery 31. Battery 31 is preferably of sufficient voltage to actuate the relay 50, thereby connecting amplifier [8 to network 54. A signal then appears on channel 19 that is the same as introduced when the solenoid is in air; i. e., in a region of zero susceptibility and zero conductivity. A two trace chart recorder (not shown), but in circuit with or substituted for meters 24 and 25, may be adjusted to zero for each trace at the beginning of the logging run.

Thus, in accordance with the present invention, there is provided in a system having a formation exploring element for producing a signal proportional to two quantities for application to phase detecting networks, a resistance and a capacitance, which by means remotely energized, are selectively coupled to the formation exploring device. Phase adjusting means are provided in circuit with the phase detectors for producing at one of the phase detectors an effect dependent entirely upon the resistor and independent or the condenser and for producing at the second of the phase detectors an effect entirely dependent upon the condenser and independent of the resistor.

accuse The formation exploringelement l0 and. bridge Ll may then be withdrawn fromv the...bottom of. the bore hole tozproduce in accordance with con;- ventional techniques a log which portrays variations in either or. both of the-:unbalancecorrrp'onents as. a functionof bore: hole: depth.

A. system for calibrating an acoustic we1l.log;-

ging deviceof: the type generically illustrated. by

thepatent: to Kean, 2,530,971,, baseduporr vibra:- tion. of amagnetostrictive cylinder,- is. illustrated in; Eig. 2.: The-Maxwell. bridge; 60 includes: inone arm a. coil 61. wound on.a. corezhaving'end pieces 62 and. 63.. The coil. 61' is. disposed coaxially of, and insidev a. nickel; tube 6.4. which is. rigidly mounted at its center by the. clamping ring 65 supported by-an; outer case 66.. Excitationof the bridges. fill. from the. source. 8.1, at. the; surface: of

the. earth. produces alternating. magnetic flux.- in

the: nickel tube 64 which, because of its magnetostrictive properties, causes itto vibrate, thus generating acoustic energy which travels. to the formations. It has been shown that although the nickel tube vibrates longitudinally, its radiation is principally radial. Variations inythe properties of the earth formations adjacent the nickel tube are reflected as impedance changes in the coil 61, which changes produce a. formation-dependent bridge unbalance signal. The latter signal is transmitted by Way of amplifier 68 to surface phase detectors 69 and of the type discussed in connection with Fig. 1.

In. order to calibrate this system for determination of the proper adjustment of the phase determining networks II and 12,. applicant provides a surface controlled relay system for coupling in the bore hole resistor I5 or condenser 16 to the coil 6|. More particularly, a second winding 11 is disposed in inductive coupling relation to the coil 6| andv is connected at one terminal to the open circuit contacts of relays 80 and 8|. The other terminal of coil 1'! is connected by way ofiresistor l5v to the armature of relay 8I- and connected by way of condenser 16. to thearmature of relay 80.. A switch 82 at thesurface. of the earth may beactuatedselectively to include battery 83 in circuit with the relays. 80 and 8| selectively to'actuate the relays for inclusion of the desired impedances or 16) in circuit with the coil 11. The resistor 15 produces an unbalance, or an added component to the bridge unbalance; of the same character as produced by the acoustic dissipative properties of the formation while the condenser introduces a component of the same nature as the components produced by the reactiveor phase shift component of the bridge unbalance.

In Fig. 3 there is illustrated a system for adding to the vibrating nickelrod 84, one end of which is shown, mechanical lumped impedances that produce eiiects of the same nature as produced by the electrical impedances 15 and 16 of Fig. 2. In both cases. the, impedances coupled to the vibrating. system produce- 1 eiiects. that are. equivalent. to aselected property of the. formation. Referring to Fig. 3 a, copper tube. 81 is rigidly fastened to the end of the nickel tube. 64. and vibrates therewith. A coil 88 encircles thecopper tube at a mid-point. One terminal of the coil 88 is connected to the armature of. the. relay 89. The normally open circuit terminal of relay 89 is connected by way of battery 90 to the other terminal of the coil 88. Upon actuation of relay 89 current flows from battery 90 through the coil 88 to establish a magnetic field, the flux lines of which pass through portions of the copper tube in which resulting eddy currents react with the; magnetic; field to: damp the: vibratory motion of: the: tube 64... Since. electrical. energy is dissipated as heat. in thezcopper; tube, the electromagnetic damping-efiectproduces an. added. com.- ponent to the bridge unbalance; signal. that. is. of the same character as is produced by'the energy dissipating of the formations.

In similar manner. there is provided means. for attaching to thev-ibratingrsystem, an added mass whichproduces a component of the samecharacter as produced by the phase shift properties of the formations. More particularly, aspring 9| supports the mass from the case (not shown) at a point generically indicated by the connection 92. Thezmass includesa yoke 93 which supports adisk S4. The disk supports. or. is integral with a. core- 9.5. around .wh-ichisdisposed a winding 96.. Anon-magnetic-rod 91: extends. through the disk 94. and is mechanically coupled to an. armature 98.. The rod 9-1 extends through the core 95 and the disc 94 and has a weight. supporting head which rests upon. disc 94. when coil 86 is not energized. Latching bars, two of which, the bars I00 and [0| are shown in Fig. 3, are pivotally supported from the disc 94 and extend vertically into the end of the copper tube 81. The latching bars I00 and HM have internal sloping cam faces for cooperation with adjacent cam faces of the'armature 98. The latching bars also have protruding ends which are normally positioned adjacent an annular groove I02 milled. inside the copper tube. Upon energization of the coil 96., the armature 91 is drawn up urging the bars I00 and [0| outward into engagement with the groove I02 in the copper tube whereupon the mass supported by spring 9! vibrates as aunit with tube 64. This produces a change in the mass-spring constant of the nickel cylinder and thus produces a component of bridge unbalance signal of the same character as the equivalent property of the formation. Coil 96 is. energized by actuation of relay H13 in a manner. similar tov that above described in connection with Eigs. 1. and12.

Thus, it willbe seen. that in alternating current systems having phase separable signals, lumped impedances may be coupled, mechanically or electrically as appropriate, to a formation exploring element to introduce into the formationdependent signal an added component that is equivalent to a selected property of the formation under investigation. and is independent of other properties. The added component then gives an operator a reliable basis for making initial adj'ustments prior to procuring a log of variations in such properties of the formations as a function of bore hole depth.

It should be observed that in order to introduce added signals that are dominately dependent upon but one property in the system of Fig. .1, it is necessary to utilize cores for the formation exploring element that are of high quality magnetic. materials, Inthe systemofFig. 1. the core was formedof laminations0.0.05" thick of ahigh quality magnetic, alloy comprised of approxi: mately equal portions of iron and nickel plus relatively small amounts of manganese and silica, known in the art under the familiar trade name Conpernik. By using such high quality core material, the impedance. of the coil was substantially entirely reactive, the phase angle between voltage and current being approximately 89. Approximately 400 turns of #16 gauge wire formed the exploring coil. Asecondary coil (coil 4-7, Fig. 1) of 30 turns of"#20 gauge wire in con- ;iunction with: a resistor 45. of 3300 ohms or a condenser 48 of 0513- microfarad introduced an added bridge unbalance signal component of sufficient magnitude to permit accurate adjustment of the phase angle by circuits 26 and 29. The system of Fig. 2 may be calibrated by observing the same considerations as above set out for the system of Fig. 1.

The construction of the elements of the system of Fig. 3 may be determined from principles of vibrating bodies well known and understood by those skilled in the art. It is to be noted that a modifying system was shown for one end of the tube in Fig. 3 only. It is to be understood that such a system may be provided for both ends of the tube.

While particular embodiments of the invention have been illustrated and described, it is to be understood that further modifications may now suggest themselves to those skilled in the art, and it is intended to cover such modifications as fall within the scope of the appended claims.

What is claimed is:

1. In a system having a formation exploring element energized from an alternating current source and movably positioned in a well bore to produce a formation-dependent signal for application to a first of a pair of input circuits of a phase detecting network, the combination which comprises an impedance having a dominant characteristic equivalent to a selected property of said formation, means including circuit completing means at the surface of the earth for coupling said impedance to said exploring element to produce an added component to said formation-dependent signal identical in character with the component of said formation-dependent signal produced by said selected property, and a circuit for transmitting from said source to the second of said input circuits a reference voltage in phase-coincidence with said added signal component.

2. In a system having a formation exploring element energized from an alternating current source and movably positioned in a well bore to produce a formation-dependent signal for application to a first of a pair of input circuits of a phase detecting network, the combination which comprises an electrical impedance having a dominant characteristic equivalent to a selected property of said formation, means including circuit completing means at the surface of the earth for inductively coupling said impedance to said exploring element to produce an added component to said formation-dependent signal identical in character with the component of said formation-dependent signal produced by said selected property, and a circuit for transmitting from said source to the second of said input circuits a reference voltage in phase-coincidence with said added signal component.

3. In a system having a formation exploring element energized from an alternating current source and movably positioned in a well bore to produce a formation-dependent signal for application to a first of a pair of input circuits of a phase detecting network, the combination which comprises a mechanical impedance having a dominant characteristic equivalent to a selected property of said formation, means including circuit completing means at the surface of the earth for mechanically coupling said impedance to said exploring element to produce an added component to said formation-dependent signal identical in character with the component of said formation-dependent signal produced by said selected property, and a circuit for transmitting from said source to the second of said-input circuits a reference voltage in phase coincident with said added signal component.

4. In a system having a formation exploring element energized from an alternating current source and movably positioned in a well bore to produce a formation-dependent signal for application to the signal input circuits of each of a pair of phase detecting networks, the combination which comprises a first impedance having a dominant characteristic equivalent to a first selected property of said formation and a second impedance having a dominant characteristic equivalent to an independent selected property of said formation, a circuit including switching means at the surface of the earth to couple in a first position said first impedance to said forma-- tion exploring element to produce a first com-' ponent of bridge unbalance signal of the same character as produced by said first selected property andto couple in said second circuit completing position said second impedance to said formation exploring element to produce a second component of bridge unbalance signal of the same character as produced by said independent selected property, a circuit interconnecting said source and reference signal input circuits of both of said detecting networks for applying to a first of said networks a reference signal in phase coincidence with said first component of bridge unbalance signal and for applying to the second of said networks a reference signal in phase coincidence with said second component of bridge unbalance signal.

5. In a system having a formation exploring element energized from an alternating current source and movably positioned in a well bore to produce a formation-dependent signal for application to the signal input circuits of each of a pair of phase detecting networks, the comination which comprises a first electrical impedance having a dominant characteristic equivalent to a first selected property of said formation and a second electrical impedance having a dominant characteristic equivalent to an independent selected property of said formation, a circuit including switching means at the surface of the earth to couple in a first position said first electrical impedance to said formation exploring element to produce a first component of bridge unbalance signal of the same character as produced by said first selected property and to couple in said second circuit completing position said second electrical impedance to said formation exploring element to produce a second component of bridge unbalance signal of the same character as produced by said independent selected property, a circuit interconnecting said source and reference signal input circuits of both of said detecting networks for applying to a first of said networks a reference signal in phase coincidence with said first component of bridge unbalance signal and for applying to the second of said networks a reference signal in phase coincidence with said second component of bridge unbalance signal.

6. In a system having an acoustic formation exploring element energized from an alternating current source and movably positioned in a well bore to produce a formation-dependent signal for application to the signal input circuits of each to a pair of phase detecting networks, the combination which comprises a first mechanical impedance having a dominant characteristic equivalent to a first selected property of said formation and a second mechanical impedance having a dominant characteristicequivalent to a difierent selected property of :said formation, a circuit including a switching means at the surface of the earth to couple in a first position said first mechanical'iinpedan'ce to said formation exploring element to produce a first component of bridge unbalance signal of the same :character as produced by said first'selected property and to couple in said second completing position said'second mechanicalimpedance to said second formation exploring element to produce a second component of bridge unbalance signal of the samecharacter as producedby said different selected property, and a circuit interconnecting said source and reference signal input circuits of both of-saiddetecting networks for applying to a first of said networks are'ference signal'in phase-coincidence with said "first component of' bridge unbalance signal and for applying to-the second of said networks a reference signal in phase-coincidence with said second component of bridge unbalance signal.

7. The combination with a bridge network including a solenoid movably positioned in a well bore and forming one bridge arm coupled to the formations and interconnected by cable means to a surface phase detecting system to which the bridge unbalance signal is applied, of a coil inductively coupled to said solenoid, a resistance and a switch in series circuit with said coil, relay means-for controlling said switch, a source of potential, means at the surface for connecting throughsaid cable means said source to said relay to close said switch and introduce into said bridge network an added unbalance signal component of the same nature as produced by the conductivity of said formations, and means for applying to said phase detecting system a reference voltage in phase-coincidence with said added unbalance signal component.

8. The combination with a bridge network including a solenoid movably positioned in a well bore and forming one bridge arm coupled to the formations and interconnected by cable means to a surface phase detecting system to which a bridge unbalance signal is applied, of a coil inductively coupled to said solenoid, a condenser and a switch in series circuit with said 0011, relay means for controlling said switch, a source of potential, means at the surface of the earth for connecting through said cable means said source to said relay to close said switch and introduce to said bridge network an added unbalance signal component of the same nature as produced by the susceptibility of the formations, and means for applying to said phase detecting system a reference voltage in phase-coincidence with said added unbalance signal component.

9. Calibration means for logging systems having a formation coupled solenoid movably positioned in a well bore and forming one arm of a bridge network which is connected to the surface of the earth means through a cable, which comprises a coil inductively coupled to said solenoid, two impedances connected to one terminal of said coil, a pair of normally open switches respectively connected to one terminal of each of said impedances, an electrical connection between the second terminal of said coil and both of said switches, a pair of relay coils for independently actuating said switches, a conductive path extending through said cable to the surface of the earth connected to one terminal of each of said relay coils, a source of potential connected to said conductive path and having a terminal positive and a terminal negative with respect to said path, a second conductive ,path extending through .said cable .into said well bore, a pair of oppositely poled rrectifierszrespectively connected between the. second terminals .of said coils and said .secondpath, and means for selectively connectingsaid second ;pathto said positive terminal and .to csaidinegative terminal forsselective flow-of current through said vrectifiers and relay coils to closesaid switches and couple one or the otherofsaid impedances .to. said solenoid.

10. Calibration ,means for logging :systems having a formation'coupled solenoid movably .positioned in a wellbore and forming one arm of a bridge network which .is connected to the surface of theeearth through a cable, which :comprises a coil inductively coupledto said-solenoid,

a resistor and a. condenser each connected to one terminal of .said coil, :a pair cf normally .open

switches respectively connected ,to said resistor and to said condenser, .anelectrical connection between thesecond terminal of said. coil andboth of said switches, a pair of relay coils for inde-:

pendently actuatingsaid switches, a conductive path extending :throughsaid cable to the surface of the earth connected tonne terminal of each o'fsaid relay coils, a source of potential connected to said vconductivepath and having :a terminal positive and a terminal negative withlrespect to said path, 'a second conductive-path extending through said cable into said we1l,bore, a pair of.

tioned in a well bore andforming one arm .of a

bridgev network which is connected to thesuriace of the earth through .a cable, which comprises a coilwound around said solenoid, aresistor and a condenser connected to oneiterminalni' saidvcoil a first normally open switch connected to said resistor, a second normally open switch connected to said condenser, an electrical connection between the second terminal of said coil and normally open contacts of said switches, a pair of relay coils for independently actuating said switches, conductor means extending through said cable to the surface of the earth connected to one terminal of each of said relay coils, a source of potential connected at its midpoint to said conductor means, a second conductor means extending through said cable into said well bore, a pair of oppositely poled rectifiers respectively connected between the second terminals of said coils and said second conductor means, and means for selectively connecting said second conductor means to the positive or negative terminal of said source of potential for flow of current through one or the other of said rectifiers to close said first or said second switch thereby to couple said resistor or said condenser to said solenoid.

12. In a system having a formation exploring element energized from an alternating current source and movably ositioned in a well bore to produce a formation-dependent signal for application to the signal input circuits of each of a pair of phase-detecting networks, the combination which comprises a coil inductively coupled to said exploring element, a resistance and a condenser connected to said coil, a pair of normally open switches respectively connected to said resistor and to said condenser, an electrical connection between the second terminal of said cell and both of said switches, a pair of relay coils for independently actuating said switches, a conductor connected to a first terminal of both of said relay coils and extending to the surface of the earth, a source of potential connected at its midpoint to said conductor, a pair of oppositely poled rectifiers connected respectively to the second terminals of said relay coils, a second conductor connected to the point intermediate said rectifiers and extending to the surface of the earth, a selector at the surface of the earth having at least two circuit completing positions, a connection between a first terminal of said source and a first of said circuit completing positions for current fiow through said selector and one of said rectifiers for closing one of said.

switches to couple said resistor to said formation exploring element to produce an added component to said formation-dependent signal equivalent to that produced by a first property of said formation, a connection between the second terminal of said source and the second of said completing positions for current flow through said selector and the second of said rectifiers to couple said condenser to said formation exploring element to produce an added component to said formation-dependent signal equivalent to that produced by a second independent property of said formation, means for applying a reference property of said medium, circuit means operable remote from said element for coupling said impedance to said exploring element to produce an added component to said medium dependent signal identical in character with the component of said medium dependent signal produced by said selected property, and circuit means for transmitting from said source to said detecting network a reference voltage in phase-coincidence with said added signal component.

14. In a system having an exploring element energized from an alternating current source to produce a signal dependent upon properties of the medium surrounding said element for application to each of a pair of detecting networks, the combination which comprises a first impedance having a dominant characteristic equivalent to a first selected property of said medium and a second impedance having a dominant characteristic equivalent to an independent selected property of said medium, means operable remote from said element for coupling the first of said impedances to said exploring element to produce a first component of the same character as produced by said first selected property and for coupling to said exploring element said second impedance to said exploring element to produce a second component of the same character as produced by said independent selected property, and circuit means interconnecting said source and said pair of detecting networks for applying to the first of said networks a reference signal in phase-coincidence with said first component and for applying to the second of said networks a reference signal in phase-coincidence with the second of said components.

CARL W. ZIMMERMAN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,812,392 Zuschlag June 30, 1931 2,535,666 Broding Dec. 26, 1950 

